1. Field of the Invention
The present invention relates generally to Q-switches for use in laser cavities and more particularly to passive Q-switches composed of members of the garnet family containing tetravalent chromium, Cr.sup.4+, dopant material.
2. Description of the Prior Art
In many modern laser applications, it is desirable for the laser cavity to produce periodic laser pulses each having a high peak power instead of more numerous, but lower powered, laser pulses. An example of such an application is the use of periodic, high power laser output pulses in the laser communication field.
In a laser cavity, a Q-switch is used to produce periodic laser pulses with high peak output powers. The function of a Q-switch is to prevent lasing in a laser oscillator while the gain medium is being pumped above the threshold value for lasing. Typically, one of three methods is used to prevent lasing: enhancement of output coupling, misalignment of resonator optics, and absorption of the lasing wavelength. These three methods for preventing lasing must then quickly change states to return the output coupling to near optimal values, align the resonator optics, and reduce the absorbance, respectively, in order to return the laser to an above-threshold condition which allows increased oscillation.
Q-switches are typically classified as either active or passive. Active Q-switches require electrical stimulation in order to function properly while passive Q-switches do not require electrical stimulation for proper operation. Therefore, passive Q-switches are desirable in many applications where additional electronic circuitry and the associated control system are not desired or may prove to be unreliable or inefficient.
Saturable absorbers, a type of passive Q-switch, utilize absorption of the lasing wavelength to prevent lasing within the laser cavity. Saturable absorbers switch states, from a state of preventing lasing to one of permitting lasing, due to transient bleaching of the absorbant material which occurs when the laser gain exceeds the net losses in the resonator, including the absorption of the Q-switch.
Typical passive Q-switches that are saturable absorbers include dyes, dye films and crystals that contain saturable color center defects. As previously discussed, these types of Q-switches are designed to exhibit an absorptivity which decreases with increasing irradiance. However, each of these Q-switches suffers from deficiencies which limit its utility. Dyes and dye films undergo degradation of the dye due to the decomposition of the long-chain dye molecules. Additionally, dye and dye film Q-switches require consistent maintenance due to decreases in the optical density of the dye which occur even while the dye is housed in the dark. A decrease in optical density of the dye alters the Q-switch properties and thus degrades the performance of the laser cavity. The index of refraction of the dyes may also change as the light intensity to which the dye is exposed increases. For example, as the laser power increases, the core of the dye through which the laser light will pass has a higher index of refraction than the exterior of the dye, thus resulting in an alteration of the Q-switch properties which degrades the performance of the laser cavity. Dyes also lack an efficient means of heat dissipation such that the thermal state of the dye may be altered during its operation in a laser cavity and thus further degrade the performance of the laser cavity.
Dye films suffer from several unique deficiencies including the flexibility of typical dye films. This flexibility necessitates the bonding of the dye film between two pieces of glass in order to prevent the dye film from bending and distorting the optical density of the film. Thus, more intricate manufacturing processes are required in order to properly bond the dye film. Furthermore, the index of refraction of dye films may vary due to changes in the temperature of the dye film so as to result in inaccurate saturable absorber behavior.
Passive Q-switches constructed from crystals containing saturable color center defects also suffer from deficiencies such as fading of the color center which in turn, causes loss of the saturable absorber characteristics which will degrade the laser performance when the Q-switch is continuously operated for several hours. Additionally, the crystals from which the Q-switch is constructed are typically rather long, such as 5 cm for a LiF crystal. Thus, the length of the crystals may restrict the use of the Q-switch in compact laser cavities.
In order to eliminate the degradation of the Q-switch properties which occur with the passage of time or exposure to laser light in the aforementioned Q-switches, inorganic solid-state materials have been utilized to provide the necessary Q-switching without experiencing the degradation noted in the other Q-switches. However, inorganic, solid-state materials have thus far been limited to use in lasing devices which incorporate both the active medium and the passive Q-switch within one device. Such a device is therefore a self-Q-switching laser. Self-Q-switching lasers suffer from several inherent limitations, including the inability to adjust the density of the dopant ions in the Q-switch to maximize its Q-switching performance without considering the corresponding effects which the dopant density change will have on the active medium's performance.
A typical self-Q-switching laser is illustrated in the article entitled, "Compact GSGG:Cr.sup.3+ :Nd.sup.3+ Laser with Passive Q Switching", published in May 1987 by A. A. Danilov, et al. in the Soviet Journal of Quantum Electronics, Volume 17(5), pages 573-574. The article discloses a flashlamp-pumped laser made of a gadolinium scandium gallium garnet (Gd.sub.3 Sc.sub.2 Ga.sub.3 O.sub.12 or GSGG) crystal activated with trivalent chromium and trivalent neodymium (GSGG:Cr.sup.3+ :Nd.sup.3+) which has both an active medium and a passibe Q-switch contained within the one device. While the laser diclosed may not suffer from the degradation of the Q-switching properties previously discussed, the device suffers from several deficiencies and limitations. One such deficiency is the limitation imposed upon a laser system in which the same device contains both the active medium and the passive Q-switch. The laser system is thus restricted to particular types of lasing elements, since only lasing elements which can accommodate the materials required for passive Q-switching may be selected. Furthermore, the wavelengths at which the laser may be operated are limited to those supported by both the active medium and the passive Q-switch. Also, as the absorption at the chosen wavelength is increased to enhance the Q-switch behavior, the laser performance is correspondingly degraded.
It would be desirable to develop a passive Q-switch, requiring no electrical contact, which does not suffer from degradation due to the passage of time or exposure to the laser light. Additionally, it would be desirable to develop a passive Q-switch which is distinct from the active medium such that the Q-switch and the lasing medium can be individually selected or altered in order to optimize the entire laser system's performance as well as to provide operation over a broader frequency range.